1. Field of the Invention
The present invention relates to an electric field measuring apparatus which uses an electro-optic material, whose optical characteristic changes according to an electric field, to measure electric field strength.
2. Related Background Art
Conventionally, as an apparatus for measuring electric field strength, there has been known an electric field measuring apparatus using an electro-optic material (e.g., Japanese Patent Application Laid-Open No. 1-1286431 and IEEE J. of Quantum Electronics, Vol. QE-22, No. 1 (1986) pp. 69-78). FIG. 1 is a configurational view of a conventional electric field measuring apparatus.
This apparatus operates as follows. First, laser light with a short pulse output from a CPM (colliding pulse mode-locked) ring dye laser 40 is divided by a half mirror into a transmitted luminous flux and a reflected luminous flux. The laser light (pumping light) reflected by the half mirror irradiates an object to be measured 10. For example, the object to be measured 10 is a photoconductive optical switch, which generates an electric pulse by photoelectric conversion when irradiated with the pumping light. This electric pulse is propagated through a transmission line, whereby an electric field is applied to an electric field sensor 20 disposed near the transmission line. On the other hand, the laser light (probe light) transmitted through the half mirror is propagated through an optical fiber 42 and then is turned into linearly polarized light by a polarizer 46. This linearly polarized light is reflected by a half mirror 50 and then is converged by a lens 62 so as to be incident on the electric field sensor 20. While traveling to-and-fro through the electric field sensor 20, probe light 30 changes its polarization state according to the electric field being applied to the electric field sensor 20. The probe light output from the electric field sensor 20 is transmitted through a phase compensating plate 54 and then is divided by an analyzer 48 into two luminous fluxes whose polarizing directions are orthogonal to each other. The intensities of the two luminous fluxes are respectively detected by photodetectors 56 and 58 so as to be measured by a lock-in amplifier 60.
For example, such an electric field measuring apparatus is used for measuring the response speed of a photoconductive optical switch shown in FIG. 2. When pumping light 2 is incident on the photoconductive optical switch 10 to which a bias voltage is being applied, an electric pulse 4 is generated by a photoelectric conversion effect. This electric pulse 4 is propagated on a transmission line 12 toward a point where the electric field sensor 20 is disposed nearby by means of a supporting needle 22, thereby applying an electric field to the electric field sensor 20. The electric field measuring apparatus detects this electric field as a change in the polarization state of the probe light 30 during its to-and-fro travel through the electric field sensor 20.
Also, the electric field measuring apparatus is used for evaluating high-speed transistors (IEEE J. of Quantum Electronics, Vol. 28, No. 10 (1992), pp. 2313-2324). When the object to be measured is in a wafer state, it is set to a semiconductor prober, and then a bias electric signal is applied thereto by way of an electric needle so as to evaluate the object to be measured.